Exotic explanation for Pioneer anomaly ruled out

The unusual trajectories of the Pioneer 10 and 11 spacecraft as they leave the solar system are not caused by any exotic new physics but by mundane thermal emissions powered by radioactive decay. That is the verdict of researchers in the US and Canada, who have compared the results of an extremely detailed computer simulation of the thermal forces on one of the craft with the same forces calculated from the trajectory of the mission. The study also suggests that the observed reduction of the extra acceleration over time is the result of how electricity is generated on board the spacecraft and distributed to its scientific instruments.

Physicists have known for more than a decade that the Pioneer 10 and 11 probes are following trajectories that cannot be explained by conventional physics. Known as the "Pioneer anomaly", both craft seem to be experiencing an extra acceleration towards the Sun as they exit the solar system that is 10 billion times weaker than the Earth's gravitational pull. Many explanations have been proposed for the origins of this anomalous acceleration, involving everything from the gravitational attraction of dark matter and modifications of Einstein's general theory of relativity to string theory and/or supersymmetry.

In 2011 a team led by Slava Turyshev of the Jet Propulsion Laboratory in California – and including Viktor Toth, Jordan Ellis and Craig Markwardt – showed that the magnitude of the acceleration is decreasing exponentially with time. Given that for both craft electricity is supplied by a radioisotope thermoelectric generator (RTGs) powered by the heat given off by the radioactive decay of plutonium – an energy source that decays exponentially with time – Turyshev and others suggested that the extra acceleration could be caused by thermal radiation being emitted from the craft in a preferred direction.

The problem with that explanation, however, is that the acceleration of the spacecraft is decaying exponentially with a half-life of about 27 years, whereas the half-life of plutonium-238 is 88 years. So to see if thermal emissions really are driving the anomaly, Turyshev, Toth and Ellis joined forces with three other researchers – Gary Kinsella, Siu-Chun Lee and Shing Lok – to create a detailed computer simulation of the thermal properties of the spacecraft and the directions in which key components emit thermal radiation.

Efficient acceleration

The simulation reveals that the two main sources of thermal emissions on the spacecraft are the RTG itself and the scientific instruments that it powers. These instruments, which are mostly mounted on the back of the spacecraft, face away from the Sun and, according to the simulations, their thermal emissions have a relatively high efficiency of accelerating the spacecraft towards the Sun. The RTG, in contrast, is mounted to one side of the main body of the spacecraft and emits thermal radiation much more evenly in all directions.

The research suggests that knowing the relative contributions of the RTG and the instruments to the anomalous acceleration is key to understanding why the observed decrease in the anomalous acceleration is faster than the decay of plutonium-238. According to Turyshev, the thermocouples at the heart of the RTGs become progressively less efficient at converting heat to electricity – and that this decay occurs with a half-life that is somewhat shorter than 88 years. As the thermocouples deteriorate, less electrical energy is supplied to the instruments, which means that the anomalous acceleration drops faster than expected from radioactive decay alone. Although more heat is dissipated by the RTG as time progresses, this has little effect on the motion of the spacecraft.

Notes and memories

According to Turyshev, the biggest challenge in developing the simulation was the "lack of precise and complete information on the spacecraft", which was designed and built more than 40 years ago. As a result, the team interviewed engineers who had built the spacecraft and still had notes and memories on the design and materials used. Also crucial to the team's success was the use of data that were beamed back to Earth during the mission. These included the temperature at several locations on the spacecraft, which allowed the team to evaluate the accuracy of its computer model and also to infer the thermal properties of some of the materials used in the satellite.

The team also performed an independent analysis of the trajectory of Pioneer 10 from which the researchers were also able to extract the relative contributions of the RTG and instruments to the anomalous acceleration. Both the thermal simulations and the trajectory analysis gave similar results, within experimental and computational errors.

It is this agreement between the thermal and trajectory studies that impresses Benny Rievers of the University of Bremen in Germany. With his colleague Claus Lämmerzahl, Rievers has also used computer modelling to show that directional thermal emissions are the likely cause of the Pioneer anomaly. "I think that we now completely understand what is going on with the spacecraft and that the anomaly is completely down to anisotropic heat radiation," says Rievers.

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13 comments

Is this thermal radiation worth considering for a final stage propulsion method on future pioneer type spacecraft? Might it be worthwhile on an unmanned interstellar voyage? Obviously the thermal source would need to be much larger.

Simple explanation

It is a simple but good detective work basedd on the knowledge of the relative positioning in the Craft, of the 238Pu-radioactive-heat source and the instruments fed by the current producd by this. The resulting anistropy in the heat emission , hence, that of the acceleration, explains nicely the preferential very weak: 10 billion times less than that of the Eath's gravittional pull, acceleration towrds the Sun.

Not very surprising

Years ago, when John Anderson first published on the Pioneer anomaly, a bunch of people (including myself) did fairly crude calculations showing that thermal effects had the right order of magnitude to explain the anomaly. After that, I really downweighted it as new physics. Occam's razor strongly suggests that, if there is a hard to model known effect of the same order of magnitude as your new physics, you probably don't have any new physics.

Photon rockets are not very effective

Is this thermal radiation worth considering for a final stage propulsion method on future pioneer type spacecraft? Might it be worthwhile on an unmanned interstellar voyage? Obviously the thermal source would need to be much larger.

The Pioneer effect is very ~ 10^-9 m/sec^2. Suppose that you could improve this by a factor of 10 (by having directed radiators, etc.). In 10 years, your new v would be = A delta_t or ~ 3 meters/sec (i.e., very little).

This is basically a photon rocket, with a very high specific impulse, but a very low momentum in the "propellant," which is why it is, although very efficient, not very effective. You would much better with an ion rocket.

To answer your question...

less electrical energy is supplied to the instruments, which means that the anomalous acceleration drops faster than expected from radioactive decay alone

I don't get it. So is the electricity reason of the anomaly - or the radiation of heat, which is indeed independent to the effectiveness of thermocouple?

The instrumentation must radiate as heat whatever electrical power it dissipates (all of it, at thermal equilibrium). But that dissipated power (provided to the instrumentation via the thermocouple) is preferentially radiated away along a vector directed away from the sun (because of the geometry of the spacecraft), whereas the radiation from the radioactive thermal source is nearly isotropic. Consequently, the thermal radiation from the instrumentation is the dominant accelerative effect, and that effect therefore decays at the rate of the thermocouple conversion efficiency decay. So you have 27=1/(1/88+1/Tausubce) => Tausubce (conversion efficiency halflife)=39 years, which seems reasonable.

It's a relief to note that dark matter has been mentioned only casually here and not seriously invoked to account for this seeming anomaly as well, and thereby lay this anomalous case conveniently to rest as 'resolved.' Far from it, I am sorry to say.Any radiation from either craft will be in the wake of the moving body, IN NET,unless intentionally designed for a frontal emission of the decay exhaust.I happened to write to Dr Anderson and Dr Turyshev following the Nature article, Support sought to investigate sluggish Pioneers, Nature 431, 494-495 (30 Sep 2004), generally supportive of their case for NASA's approval of their grant application to study further "to help resolve the uncertainty by reanalysing data from the first decade of the Pioneer missions."To make my comment short here, I shall leave with that letter for your kind perusal and to draw your own conclusions - which could potentially resolve also the dark matter problem across the cosmos that's still beguiling us to this day.www.sittampalam.net…PioneerAnomaly.htm

A confirmation of previous work.

Dear Sir,

Regarding your post about a reported thermal solution to Pioneer Anomaly riddle, we would like to call your attention that a thermal solution has been obtained in March of last year in our paper "Modelling the reflective thermal contribution to the acceleration of the Pioneer spacecraft" (arxiv.org…1103.5222), which unfortunately your post fails to mention.

In our paper we use a relatively simple yet effective approach to demonstrate that the thermal effects explain any Pioneer anomaly within the knowledge of the effect, and the result is robust to the uncertainty of the parameters of the spacecraft.

Actually, the paper from the German team that you mentioned appeared after our paper. And should be said that the novel paper from the JPL team that motivated your post is an elaboration of the German team approach, together with a parametric analysis much in line with the one of our paper.

After a long and useless battle with referees that did all they could to delay the publication of our work our paper is about to be published in Physics Letters B (www.sciencedirect.co…03702693), essentially as it appeared in March 2011, but by cosmetic details.

what about the power regulation - there must have been a power regulator e.g. as the thermo ionic generators drop in efficiency, there must be a linear regulation system to compensate for the drop in bus voltage. Are the regulators in the RTG or with the electronics at the back of the dish ? Regulator losses (heat dissipation) would be highest early in mission and lower later. The regulator would account for a significant amount of dissipated heat. However, the question is, does the total regulator (and how it is mechanically heat sunk) and electronics dissipation profile "match" the anomoly curve ?

less electrical energy is supplied to the instruments, which means that the anomalous acceleration drops faster than expected from radioactive decay alone

I don't get it. So is the electricity reason of the anomaly - or the radiation of heat, which is indeed independent to the effectiveness of thermocouple?

The instrumentation must radiate as heat whatever electrical power it dissipates (all of it, at thermal equilibrium). But that dissipated power (provided to the instrumentation via the thermocouple) is preferentially radiated away along a vector directed away from the sun (because of the geometry of the spacecraft), whereas the radiation from the radioactive thermal source is nearly isotropic. Consequently, the thermal radiation from the instrumentation is the dominant accelerative effect, and that effect therefore decays at the rate of the thermocouple conversion efficiency decay. So you have 27=1/(1/88+1/Tausubce) => Tausubce (conversion efficiency halflife)=39 years, which seems reasonable.